This invention relates to improvements in exhaust gas purifying system and method, and more particularly to the exhaust gas purifying system and method for removing five noxious components, NOx, HC, CO and PM (including SOF and soot) in exhaust gas such as one discharged from a diesel engine, at high efficiencies.
In recent years, lean-burn engines which are mainly operated on air-fuel mixture having air-fuel ratios richer than a stoichiometric value have been spread from the view points of improving fuel economy and reducing an amount of emission of carbon dioxide. Attention on the lean-burn engines have been paid particularly for diesel engines because of a high fuel economy characteristics in the lean-burn engines. However, exhaust gas of the diesel engines (referred to as “diesel exhaust gas”) is high in oxygen content as compared with conventional gasoline-fueled engines which are operated on air/fuel mixtures having air/fuel ratios around the stoichiometric value, so that purification of nitrogen oxides (NOx) becomes insufficient in case of using a conventional three-way catalyst. Further, the diesel engines are low 50 to 100° C. in exhaust gas temperature as compared with the gasoline-fueled engines. In addition, exhaust gas of the diesel engines contains also particulate matter (PM), and therefore is difficult to be purified by using exhaust gas purifying catalysts of the conventional structures. Furthermore, in recent years, fuel economy improvements have been accomplished providing such a tendency that temperatures of exhaust gas of the diesel engines are further lowered so that discharging exhaust gas having temperatures not higher than 200° C. frequently occurs.
Under such circumstances, it has been desired to develop a catalyst for removing noxious components contained in exhaust gas from the diesel engines at high efficiencies. As exhaust gas purifying catalysts for diesel engines, oxidizing catalysts have been conventionally used in which platinum is carried on an inorganic substrate such as alumna or the like. However, although the oxidizing catalysts mainly function to oxidize or remove carbon monoxide (CO) and hydrocarbons (HC) and may oxidize or remove soluble organic fraction (SOF) in the particulate matter to some extent, oxidation or removal of carbon particle (dry soot) as solid particle cannot be effectively accomplished. Additionally, it has been pointed out that when the carried amount of platinum (Pt) serving as an active catalyst component in the exhaust gas purifying catalyst is increased in order to improve an oxidizing effect of the exhaust gas purifying catalyst particularly under low temperature conditions of not higher than 200° C., a large amount of sulfate is produced upon an increase in exhaust gas temperature, which is disadvantageous.
In order to suppress the baneful influence of sulfate and effectively remove the noxious components, it has been proposed to use a catalyst includes a substrate formed of titania to which S component hardly adheres, and a noble metal carried on the substrate, as disclosed in “TOYOTA Technical Review Vol. 47, No. 2, pages 108 to 113 (November 1997) and Japanese Patent Provisional Publication No. 10-180096, which also show the effectiveness of addition of zeolite carrying Pt. In such a conventional proposition, it is pointed out that Pt/zeolite adsorbs SOF and reforms SOF even at relatively low temperatures such as around 150° C., thereby improving the combustibility of SOF. However, in the conventional proposition, evaluation of the catalyst is carried out by using n-hexadecane as an imitation component for SOF, and therefore evaluation of the catalyst on actual gas containing high boiling point components having 20 or more in number of C is not accomplished while combustibility of dry soot (carbon) is not apparent. Accordingly, the effectiveness of the catalyst in the conventional proposition is not apparent in case of being used in a low exhaust gas temperature range of not higher than 200° C. for a long time. Additionally, in the above Review, although the reducing (removing) efficiency to NOx is confirmed during a vehicle cruising mode, the confirmed reducing efficiency is not necessarily sufficient. It will be understood that it is desired to remove at a high efficiency NOx, CO (carbon monoxide), HC (hydrocarbons) and PM in order to accomplish purification of diesel exhaust gas at a high efficiency.
To remove PM, it is essential to apply a filtering technique, and therefore using a porous sintered body or fibrous filter formed of cordierite or silicon carbide has been proposed. It has been also proposed that the fibrous filter is formed of a variety of materials such as alumina and silica. In this connection, a diesel particulate filter (DPF) formed of silicon carbide fiber was proposed in previously printed matters of a scientific lecture in Society of Automotive Engineers of Japan No. 103-98 (an autumn meeting in 1998). However, this fibrous filter is required to be provided with a heater for thermally removing PM trapped on the fibrous filter for the purpose of regenerating the fibrous filter. Thus, this proposition requires a complicated system and therefore is difficult to be applied to small-sized automotive vehicles.
As a method of regenerating the filter without using the heater, the following method has been proposed: A catalyst containing Pt as a main catalyst component is disposed upstream of a filter formed of a ceramic, in which NO in exhaust gas is converted into NO2 having a strong oxidizing ability, followed by combusting PM trapped on the filter under the strong oxidizing ability of NO2. Such a proposition is disclosed in Japanese Patent Provisional Publication No. 1-318715; J. P. Warren et al., “Effects on after-treatment on particulate matter when using the Continuously Regenerating Trap”, ImechE 1998 S491/006; and B. Carberry et al., “A focus on current and future particle after-treatment systems”, ImechE 1998 S491/007. This method uses reactions among components contained in exhaust gas thereby to continuously combust PM trapped on the filter, and therefore is called a continuously regenerating trap. However, in the present status, there exist limitations for applying the above method, so that a range within the limitations is relatively narrow. For example, a temperature range in which conversion of NO into NO2 can be made is so limited as to be difficult to occur in a temperature condition of not higher than 200° C. Additionally, it is difficult to obtain a necessary amount of NO2 to be required for combustion of PM, while the problem of poisoning with S in exhaust gas may arise.
In addition, a method of combusting and removing PM trapped on a filter under an intermittent thermal control has been proposed in Japanese Patent Provisional Publication No. 7-189656, in which incombustible PM and combustible PM are separately trapped so as to improve the efficiency of regeneration of the filter upon combustion. However, this method requires to changeover the flow direction of exhaust gas in accordance with operating conditions of an internal combustion engine, which is complicated. In this method, combustion heat generated at an upstream trap for the combustible PM (rich in SOF) is supplied to a downstream trap for the incombustible PM (rich in dry soot) so that the supplied heat is used for regeneration of the downstream trap. The upstream trap for combustible PM serves similarly as a catalyst for the purpose of accomplishing warming-up, as disposed in Japanese Patent Provisional Publication No. 61-112716. In other words, the above method in Japanese Patent Provisional Publication No. 7-189656 uses an oxidizing catalyst similarly to conventional methods; however, the conventional methods are not provided with a measure for preferentially combusting SOF in an oxidizing catalyst, and additionally no consideration is taken on adhesion of soot so that it is not apparent as to whether the oxidizing catalyst is durable or not in a low exhaust gas temperature condition and in use for a long time.
Further, in case of using a complicated system for raising an exhaust gas temperature under controlling a throttle valve of an internal combustion engine, with the above method, the following problems will arise: First, it is unclear as to whether heat generation in the upstream trap having the function of an oxidizing catalyst can supply a sufficient quantity of heat to completely combust incombustible PM on the downstream trap. Second, there are apprehensions of occurrence of a thermal deterioration of catalyst components, and of disadvantages due to control of the throttle valve. Additionally, it is not apparent as to whether the traps is endurable or not to use for a long time.
Similarly, Japanese Patent Provisional Publication No. 8-312331 discloses a method using an upstream oxidizing catalyst and a downstream filter, in which light oil or fuel is supplied to and burnt at the upstream oxidizing catalyst so as to raise an exhaust gas temperature, followed by combusting soot on the downstream filter under the effect of high temperature exhaust gas from the upstream oxidizing catalyst. Also in this case, there are apprehensions of thermal deterioration of catalyst components and fuel economy degradation due to supply of fuel.
For the purposes other than exhaust gas purification of automotive vehicles, a fibrous filter on which a variety of catalyst components, zeolite and the like are carried has been proposed in Japanese Patent Provisional Publication No. 11-290624. More specifically, the fibrous filter includes two or more fibrous layers which are laminated to form a filter material. Functional agents such as silicon oxide, activated carbon, zeolite, clay and the like are carried in the filter material. The functional agents are powder-like and contained in the filter material, and has an average particle size larger than the average pore size of the fibrous layer of the filter material. The filter material is formed of polypropylene so as to exhibit a removal performance of ammonia gas. However, no consideration is taken on removal of particle such as particulate matter discharged from an automotive vehicle engine and on a continuous regeneration of the filter. Additionally, heat resistance, particulate trapping and combustion characteristics of the filter are unclear.
Japanese Patent Provisional Publication No. 10-290921 discloses a deodorizing catalytic filter which includes a corrugated fibrous ceramic sheet on which zeolite, manganese (Mn), copper (Cu), platinum (Pt), palladium (Pd), silver (Ag) and the like are carried. However, no consideration is taken on removable of particles and on a continuous regeneration of the filter. Additionally, applicability of the filter to exhaust gas of an automotive vehicle engine is unclear.